In recent years, a number of different applications have arisen in which digital signals representing data are processed and the processed signals are then converted to analog signals. For example, such applications have included the transmission of television signals through coaxial lines to homes. In such systems, the digital data is converted to analog signals and the analog signals are then transmitted through coaxial lines to homes of subscribers. Other applications are in microwave links to satellite communications.
In a number of the different applications involving the processing of digital data and the conversion of the processed digital data to analog signals, the digital data is provided at a variable frequency or rate and the analog signals are provided at a fixed frequency different from the variable frequency or rate. For example, the digital data may be provided in the range of approximately 10-40 megabits per second and the analog signals may be sampled at a fixed frequency of approximately 120 megahertz.
In the above example, the digital signals in the range of 10-40 megabits/second are converted to an intermediate frequency having a fixed value. For example, the digital signals in the range of 10-40 megabits may be converted to signals at an intermediate frequency of approximately 5 megahertz. The signals at the intermediate frequency are then used to modulate the signals at the fixed sampling frequency of approximately 120 megahertz.
As will be seen from the above discussion, a considerable range of frequencies (e.g. 10-40 megabits/second) has to be converted to a single fixed intermediate frequency (e.g. 5 megahertz). This is not easy. If the conversion is not accurate, the signals at the intermediate frequency jitter. When the signals illustratively provide television information, the jitter produces a significant deterioration in the quality of the television image.